A three way catalyst of Pt, Rh, Pd, or the like has been generally used as a catalyst for the purification of an automobile exhaust gas. In order for the catalyst to function, the catalyst should be heated to a temperature of about 350.degree. to 400.degree. C. For catalytic converters in current extensive use, a catalyst is supported on a ceramic carrier or a metallic carrier, and the catalyst is heated and activated by taking advantage of heat energy of the exhaust gas per se. This is disadvantageous in that much time is taken for the catalyst to function after the start of an engine, resulting in poor conversion efficiency immediately after the start of the engine.
Accordingly, in the background of tightening of exhaust gas regulations in recent years, catalytic converters having an excellent conversion efficiency immediately after the start of an engine, i.e., at the time of start of a cold engine, have been desired in that art. One method for meeting this demand comprises forming an insulating film on at least one of a flat sheet (a thin flat sheet) and a corrugated sheet (a thin corrugated sheet) in a honeycomb structure constituting a metallic carrier, winding the flat sheet and the corrugated sheet in a mutually electrically insulated state, and conducting energization in a spiral direction to generate Joule heat which forcibly heats the catalyst, thereby shortening the time taken for the temperature of the catalyst to reach a value at which the catalyst functions.
PCT Publication No. 3-500911 published in Japan discloses a metallic carrier, having a resistance value suitable for electric heating, wherein an insulating layer is provided in a honeycomb structure comprising a flat sheet and a corrugated sheet. One example of the structure disclosed in the above publication is shown in FIG. 1. In such a structure, insulating layers 180 are disposed at proper intervals in a honeycomb structure 151 to form a current path as indicated by an arrow. The electric power consumed in the current path is regulated to 50 to 500 W which is an electric power that can be supplied from a battery for automobiles without difficulties. However, the whole honeycomb structure is electrically heated as in the above case where an insulating film is provided on the flat sheet and the corrugated sheet, so that much time is taken for the temperature to reach a desired value.
Japanese Unexamined Patent Publication (Kokai) No. 5-59939 discloses a metallic carrier having such a construction that a heater is buried in a honeycomb structure. In this construction, only the buried heater is electrically heated. In fact, however, a combustion reaction represented, for example, by 2CO+O.sub.2 =2CO.sub.2 occurs upon the initiation of a catalytic reaction. This reaction is an exothermic reaction and generates a large quantity of heat. Specifically, when only a portion of the honeycomb structure is heated to a temperature at which the catalyst supported on the metallic carrier functions, the occurrence of a reaction in the heated portion enables the other portions to be spontaneously and rapidly heated by virtue of the heat of reaction. In this case, the electrically heated volume is smaller than that in the above-described prior art, so that when the same electric power is applied, the temperature of the catalyst in the heated portion reaches, in a shorter time, a value at which the catalyst functions.
Although the method described in Japanese Unexamined Patent Publication (Kokai) No. 5-59939 has an excellent function, it has a drawback that in the production of the metallic carrier, the process is complicated because after a honeycomb having a hole or a groove for burying a heating heater therein is produced, a heater fabricated so as to have a shape corresponding to the hole or groove should be buried in the hole or groove.
The present invention solves the above problems particularly in a preheat type metallic carrier.
Specifically, an object of the present invention is to strongly join a heat resisting alloy having on its surface an insulating film (hereinafter referred to as "insulated heat resisting alloy").
Another object of the present invention is to impart good electric conductivity to a joint of an insulated heat resisting alloy.
Another object of the present invention is to provide a preheat type metallic carrier, for a catalyst for the purification of an exhaust gas, having a honeycomb structure comprising a flat sheet and a corrugated sheet, at least one of which is made of an insulated heat resisting alloy, which metallic carrier enables the inside of the honeycomb structure to be energized through a very simple structure.
A further object of the present invention is to make it possible for a metallic carrier to be preheated by energizing a part of the inside of the above honeycomb structure.
A further object of the present invention is to sufficiently join the flat sheet and the corrugated sheet to each other in the above honeycomb structure.